Devices which measure and count particles in a fluid are well known. Such devices are employed, for example, by semiconductor wafer manufacturers to monitor the extent of airborne particulate matter in a clean room. Pharmaceutical manufacturers employ such devices for the detection and control of foreign particles.
One method of particle detection is light blockage particle counting, or light obscuration. Light obscuration sensors work on the principle of the casting of a shadow onto a photodetector as a flow of particle-laden fluid is directed through a light beam generated by an incandescent lamp. A more sensitive method is the light scattering method. As a particle passes through a light beam, the particle scatters light. For a stationary particle the amount of scattered light is a function of the particle size, the wavelength and intensity of the incident light, and the difference between the light scattering properties of the particle and the surrounding medium. A laser source may be used to generate the light beam and the scattered light is sensed by a detector which provides readable signals indicative of particle size.
In employment of the light scattering method other factors must be considered in the detection of particles which are in motion, rather than, stationary. In particle-detection applications such as clean room monitoring, the flow rate of a given volume is typically a standard rate, for example, one cubic foot per minute. U.S. Pat. No. 4,798,465 to Knollenberg teaches the benefits of a sample flow rate of one cfm. However, the velocity of a sample flow determines the time in which a particle remains within the view volume of a detection device. The quantity of light which a given-sized particle will scatter while in the view volume is inversely proportional to particle velocity and directly proportional to particle size. Increasing the velocity of a particle by a factor of two results in a halving of the time span in which the particle travels through the view volume, thereby decreasing the quantity of light scattered by the particle.
Typically there is a direct relation between counter sample rate, counter size, and counter cost. For example, Knollenberg teaches a detection device having a sensing region for detecting and counting particles which scatter the light from a laser beam. A linear array of detectors is used, with each detector monitoring a different portion of the single sensing region. The patent teaches that use of a linear array of detectors enables sensing of particles of 0.1 micron even in the high background of molecular scattering associated with a flow rate of 1 cfm.
An object of the present invention is to provide an apparatus for detecting and counting particles with a relatively high sensitivity but in a cost-efficient and compact manner.